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Most of the mainstream EV's only use a Contactor (or two). They're spring loaded and usually have a magnet that helps quench the DC arcing. The EV200 and LEV200 I use in my systems can break 2000A at 320VDC, which is significant.

I've not designed a system with an extra circuit disconnect, other than a battery plug so I can disconnect. No emergency switch/e-stop.

I think the big thing is, use something that can safely disconnect without welding the contacts in the process. So if you use a contactor that can only break 300A, and your controller can have a battery current of like 500A, then you can't disconnect if there's an emergency. If you used an EV/LEV200, you can.

I don't disagree that a mechanical/manual disconnect is safe, but I think if you do design using the proper components, you will also be safe, especially with AC controllers. If AC controllers fail shorted, the motors don't go into runaway, they just stop, open the contactor or blow the fuse, or both. With Series wound or PM DC motors, the controllers can potentially fail shorted, and go into runaway.

Just make sure your fuse is rated for less current than the breaking amp capacity of the contactor you're using.

Sorry, first, not many of us here are engineers or builders at Tesla (though one friend from here is, and he runs a pack cut-off) or any "mainstream" not garage-build-by-not-EV-engineer company. Also, Tesla is a ground-up build by really amazing talent, not a conversion. But if you feel your design is not ever going to fail catastrophically in a way you didn't anticipate, god love ya, or as my mother was fond of saying, go ahead, nothing I say is going to change your mind. I, for one, have seen several incidents that I could never have anticipated, both with my own builds and others' builds - not necessarily pack-cutout related, but still. (My father's comment was "if that didn't teach you anything, there's nothing I can say that will." )

Second, what is the issue? I don't buy the complexity or fault source argument. If you can build in a simple $40 pack cutoff, why wouldn't you do it on your, what, first or second build? If you have as many designs or builds as Travis under your belt, then yeah, sure, do whatever, although I still would toss it in. It seems like quick, easy piece of mind to me.

edit: Now that I think about it, the one case where I use it constantly? Every single time I do ANYthing to the wiring of the bike. That convenience is worth the $40 right there.

If I have any more comments along these lines, yeah, I'll put it in Fight Club.

(OK, back down to TWO emoji, but including quotes from BOTH parental units... hmmm.)

To each his own .. I'm not gonna lose any sleep over it.
I'm really not worried the least bit over it. If I need to do any work on the harness, I can unbolt the negative terminal cable in less than a minute.

I am not trying to give you a hart time, but IMO Your "playin' with fire".

If (1) police officer, fireman, paramedic or even an average citizen gets hurt "fried"' while trying to help in a crash, it'll negatively affect the entire EV industry, community & movement.

I have heard of situations where first responders were reluctant (scared) to help someone in a wrecked vehicle because they saw it was an "electric conversion".

If it got around that there are "rogue" unswitchable DIY electric conversions around...

But, mainly it's for "your safety", your house has a "main" cut-off switch & industrial machines have "main" cut-off switches to allow (no to dictate) that the system is completely off (dead) & therefor safe(er) to touch/work on.

Mostly things right off the pack. The thing that comes to mind first is wear/rubbing on the cable before the fuse. It depends on where you locate your pack, and how the leads are configured, of course. Dropping your bike in a puddle, stream or whatever. Malfunctions/leaks of devices in the low-voltage circuit. Dumping the bike - being able to shut off power if the throttle is stuck on (happened to me more times on gas bikes than I care to remember). I've even had my pack overheating for no apparent reason, and simply reached down and cut power so I could coast to a stop and figure out WTF was going on. I had a friend jokingly grab and twist my throttle while I was sitting on the bike, with it turned on. It was fairly exciting. I destroyed a pack by leaving the switch on, due to the Alltrax sapping the power - I actually put a "Power On" LED on the bars to make sure I'd never do that again. Kidproofing the bike - I've done several shows where kids are all over the thing, and did my best to keep them from killing themselves, but had the pack disconnected via the switch as well.

I could keep going. Unexpected things tend to happen the longer you build and ride these things.

Your call, of course, but you asked for reasons for the switch, and I'm givin' em to you.

If the phone has enough charge to turn the bike on, that's all that's needed to get home. The bike will continue working with a dead phone once it's turned on. The Cycle Analyst provides a back-up speedometer.

Do you have back up plans (plan B) built into your design?

Hypothetical:
What if you ride your bike to your buddies house for, oh lets say a BBQ, you drop your phone & it gets busted.

Will your secret button allow you to ride 'er home?

Also, do you carry a spare fuse "just in case"?

I read where Ben Nelson got stranded before by just "blowing the fuse" & not having a spare, with him.

I don't carry a 500 amp main fuse with me, but I do carry the smaller assortment of 3,5 and 10 amp bus fuses for the 12v system.
I figure if the 500a main pops, then there is something seriously wrong and I wouldn't want to just pop a new one in without diagnosing first.

Ya, the button will make it work without the phone, that was intentional, and its been tested several times, and works like a charm. It would make it easier to steal, so sshhhhh.. don't tell nobody! But it is very very hard to get at and you wouldn't know it was there if nobody told ya. But if a thieve really wants it, a flat bed trailer or truck is all they need.